In order to place a power transformer/reactor according to the invention in its proper context and hence be able to describe a new approach in accordance with the invention in addition to the advantages afforded by the invention with respect to the prior art, a relatively complete description of a power transformer as it is currently designed will first be given below as well in addition to the limitations and problems which exist when it comes to calculations, design, insulation, earthing, manufacture, use, testing, transport, etc., of these transformers.
With respect to the above-mentioned, there is a comprehensive literature describing transformers in general, and more particularly, power transformers. Reference may be made, for example, to the following:
The J & P Transformer Book, A Practical Technology of the Power Transformer, by A. C. Franklin and D. P. Franklin, published by Butterworths, edition 11, 1990.
Regarding the internal electrical insulation of windings, etc., the following can be mentioned: Transformerboard, Die Verwendung von Transformerboard in Grossleistungstransformatoren by H. P. Moser, published by H. Weidman AG, CH-8640 Rapperswil.
From a purely general point of view, the primary task of a power transformer is to allow exchange of electric energy between two or more electrical systems of, usually, different voltages with the same frequency.
A conventional power transformer comprises a transformer core, in the following referred to as a core, often of laminated oriented sheet, usually of silicon steel. The core comprises a number of core limbs, connected by yokes which together form one or more core windows. Transformers with such a core are often referred to as core transformers. Around the core limbs there are a number of windings which are normally referred to as primary, secondary and control windings. As far as power transformers are concerned, these windings are practically always concentrically arranged and distributed along the length of the core limbs. The core transformer usually has circular coils as well as a tapering core limb section in order to fill up the window as effectively as possible.
In addition to the core type transformer there is so-called shell-type transformer. These are often designed with rectangular coils and a rectangular core limb section.
Conventional power transformers, in the lower end of the above-mentioned power range, are sometimes designed with air cooling to dissipate the heat from inherent losses. For protection against contact, and for possibly reducing the external magnetic field of the transformer, it is often provided with an outer casing provided with ventilation openings.
Most of the conventional power transformers, however, are oil-cooled. One of the reasons for this is that the oil has an additional very important function as insulating medium. An oil-cooled and oil-insulated power transformer is therefore surrounded by an external tank on which, as will be clear from the description below, very high demands are placed.
Usually, means for water-cooling of the oil are provided.
The following part of the description will for the most part refer to oil-filled power transformers.
The windings of the transformer are formed from one or several coils connected in series built up of a number of turns connected in series. In addition, the coils are provided with a special device to allow switching between the taps of the coils. Such a device may be designed for tapping with the aid of screw joints or more often with the aid of a special switch which is operable in the vicinity of the tank. In the event that switching can take place for a transformer under voltage, the changeover switch is referred to as an on-load tap changer whereas otherwise it is referred to as a deenergized tap changer.
Regarding oil-cooled and oil-insulated power transformers in the upper power range, the contacts of the on-load tap changers are placed in special oil-filled containers with direct connection to the transformer tank. The contacts are operated purely mechanically via a motor-driven rotating shaft and are arranged so as to obtain a fast movement during the switching when the contact is open and a slower movement when the contact is to be closed. The on-load tap changers as such, however, are placed in the actual transformer tank. During the operation, arcing and sparking occur. This leads to degradation of the oil in the containers. To obtain less arcs and hence also less formation of soot and less wear on the contacts, the on-load tap changers are usually connected to the high-voltage side of the transformer. This is due to the fact that the currents which need to be broken and connected, respectively, are smaller on the high-voltage side than if the on-load tap changers were to be connected to the low-voltage side. Failure statistics of conventional oil-filled power transformers show that it is often the on-load tap changers which give rise to faults.
In the lower power range of oil-cooled and oil-insulated power transformers, both the on-load tap changers and their contacts are placed inside the tank. This means that the above-mentioned problems with respect to degradation of the oil because of arcing during operation, etc., affect the whole oil system.
From the point of view of applied or induced voltage, it can broadly be said that a voltage which is stationary across a winding is distributed equally onto each turn of the winding, i.e., the turn voltage is equal on all the turns.
From the point of view of electric potential, however, the situation is completely different. One end of a winding is usually connected to earth. This means, however, that the electric potential of each turn increases linearly from practically zero in the turn which is nearest the earth potential up to a potential in the turns which are at the other end of the winding which correspond to the applied voltage.
This potential distribution determines the composition of the insulation system since it is necessary to have sufficient insulation both between adjacent turns of the winding and between each turn and earth.
The turns in an individual coil are normally brought together into a geometrical coherent unit, physically delimited from the other coils. The distance between the coils is also determined by the dielectric stress which may be allowed to occur between the coils. This thus means that a certain given insulation distance is also required between the coils. According to the above, sufficient insulation distances are also required to the other electrically conducting objects which are within the electric field from the electric potential locally occurring in the coils.
It is thus clear from the above-mentioned description that for the individual coils, the voltage difference internally between physically adjacent conductor elements is relatively low whereas the voltage difference externally in relation to other metal objects—the other coils being included—may be relatively high. The voltage difference is determined by the voltage induced by magnetic induction as well as by the capacitively distributed voltages which may arise from a connected external electrical system on the external connections of the transformer. The voltage types which may enter externally comprise, in addition to operating voltage, lightning overvoltages and switching overvoltages.
In the current conductors of the coils, additional losses arise as a result of the magnetic leakage field around the conductor. To keep these losses as low as possible, especially for power transformers in the upper power range, the conductors are normally divided into a number of conductor elements, often referred to as strands, which are connected in parallel during operation. These strands must be transposed according to such a pattern that the induced voltage in each strand becomes as equal as possible and so that the difference in induced voltage between each pair of strands becomes as small as possible for internally circulating current components to be kept down at a reasonable level from the loss point of view.
When designing transformers according to the prior art, the general aim is to have as large a quantity of conductor material as possible within a given area limited by the so-called transformer window, generally described as having as high a fill factor as possible. The available space shall comprise, in addition to the conductor material, also the insulating material associated with the coils, partly internally between the coils and partly to other metallic components including the magnetic core.
The insulation system, partly within a coil/winding and partly between coils/windings and other metal parts, is normally designed as a solid cellulose- or varnish-based insulation nearest the individual conductor element, and outside of this as solid cellulose and liquid, possibly also gaseous, insulation. In this way, windings with insulation and possible support parts represent large volumes which will be subjected to high electric field strengths which arise in and around the active electromagnetic parts of the transformer. In order to predetermine the dielectric stresses which arise and achieve a dimensioning with a minimum risk of breakdown, good knowledge of the properties of insulating materials is required. It is also important to achieve such a surrounding environment that it does not change or reduce the insulating properties.
The currently predominant insulation system for high-voltage power transformers comprises cellulose material as the solid insulation and transformer oil as the liquid insulation. The transformer oil is based on so-called mineral oil.
The transformer oil has a dual function since, in addition to the insulating function, it actively contributes to cooling of the core, the winding, etc., by removal of the loss heat of the transformer. Oil cooling requires an oil pump, an external cooling element, an expansion vessel, etc.
The electrical connection between the external connections of the transformer and the immediately connected coils/windings is referred to as a bushing aiming at a conductive connection through the wall of the tank which, in the case of oil-filled power transformers, surrounds the actual transformer. The bushing is often a separate component fixed to the tank wall and is designed to withstand the insulation requirements being made, both on the outside and the inside of the tank, while at the same time it should withstand the current loads occurring and the resulting current forces.
It should be pointed out that the same requirements for the insulation system as described above regarding the windings also apply to the necessary internal connections between the coils, between bushings and coils, different types of switches and the bushings as such.
All the metallic components inside a power transformer are normally connected to a given earth potential with the exception of the current-carrying conductors. In this way, the risk of an unwanted, and difficult-to-control, potential increase as a result of capacitive voltage distribution between current leads at high potential and earth is avoided. Such an unwanted potential increase may give rise to partial discharges, so-called corona, which may be revealed during the normal acceptance tests, which partially are performed, compared with rated data, increased voltage and frequency. Corona may give rise to damage during operation.
The individual coils in a transformer must have such a mechanical dimensioning that they may withstand any stresses occurring as a consequence of currents arising and the resulting current forces during a short-circuit process. Normally, the coils are designed in such a way that the forces arising are absorbed within each individual coil, which in turn may mean that the coil cannot be dimensioned optimally for its normal function during normal operation.
Within a narrow voltage and power range of oil-filled power transformers, the windings are designed as so-called helical windings. This implies that the individual conductors mentioned above are replaced by thin sheets. Helical-wound power transformers are manufactured for voltages of up to 20-30 kV and powers of up to 20-30 MW.
The insulation system of power transformers within the upper power range requires, in addition to a relatively complicated design, also special manufacturing measures to utilize the properties of the insulation system in the best possible way. In order to obtain a good insulation to be obtained, the insulation system shall have a low moisture content, the solid part of the insulation shall be well impregnated with the surrounding oil and the risk of remaining “gas” pockets in the solid part must be minimal. To ensure this, a special drying and impregnating process is carried out on a complete core with windings before it is lowered into a tank. After this drying and impregnating process, the transformer is lowered into the tank which is then sealed. Before filling of oil, the tank with the immersed transformer must be emptied of all its air. This is done in connection with a special vacuum treatment. After carrying this out the tank is filled with oil.
In order to obtain the promised service life, etc., almost absolute vacuum is required during the vacuum treatment. This thus presupposes that the tank which surrounds the transformer is designed for full vacuum, which entails a considerable consumption of material and manufacturing time.
If electric discharges occur in an oil-filled power transformer, or if a local considerable increase of the temperature in any part of the transformer occurs, the oil disintegrates and gaseous products dissolve in the oil. The transformers are therefore usually provided with monitoring devices for detection of gas dissolved in the oil.
For weight reasons large power transformers are transported without oil. On-site installation of the transformer at the customer requires, in turn, renewed vacuum treatment. In addition, this is a process which, furthermore, has to be repeated each time the tank is opened for some repair work or inspection.
It is obvious that these processes are very time-consuming and cost-demanding and constitute a considerable part of the total time for manufacture and repair while at the same time requiring access to extensive resources.
The insulating material in conventional power transformers constitutes a large part of the total volume of the transformer. For a power transformer in the upper power range, oil quantities in the order of several tens of cubic meters of transformer oil are not unusual. The oil which exhibits a certain similarity to diesel oil is thinly fluid and exhibits a relatively low flash point. It is thus obvious that oil together with the cellulose constitutes a non-negligible fire hazard in the case of unintentional heating, for example at an internal flashover and a resulting oil spillage.
It is also obvious that, especially in oil-filled power transformers, there is a very large transport problem. Such a power transformer in the upper power range may have a total oil volume of several decades of cubic meters and may have a weight of up to several hundred tons. It is realized that the external design of the transformer must sometimes be adapted to the current transport profile, i.e., for any passage of bridges, tunnels, etc.
A short summary of the prior art with respect to oil-filled power transformers follows hereafter in which both its limitations and problem areas will be described:
An oil-filled conventional power transformer                comprises an outer tank which is to house a transformer comprising a transformer core with coils, oil for insulation and cooling, mechanical support devices of various kinds, etc. Very large mechanical demands are placed on the tank, since, without oil but with a transformer, it shall be capable of being vacuum-treated to practically full vacuum. The tank requires very extensive manufacturing and testing processes and the large external dimensions of the tank also normally entail considerable transport problems;        normally comprises a so-called pressure-oil cooling. This cooling method requires the provision of an oil pump, an external cooling element, an expansion vessel and an expansion coupling, etc.;        comprises an electrical connection between the external connections of the transformer and the immediately connected coils/windings in the form of a bushing fixed to the tank wall. The bushing is designed to withstand any insulation requirements made, both regarding the outside and the inside of the tank;        comprises coils/windings whose conductors are divided into a number of conductor elements, strands, which have to be transposed in such a way that the voltage induced in each strand becomes as equal as possible and such that the difference in induced voltage between each pair of strands becomes as small as possible;        comprises an insulation system, partly within a coil/winding and partly between coils/windings and other metal parts which is designed as a solid cellulose- or varnish-based insulation nearest the individual conductor element and, outside of this, solid cellulose and a liquid, possibly also gaseous, insulation. In addition, it is extremely important that the insulation system exhibits a very low moisture content;        comprises as an integrated part an on-load tap changer, surrounded by oil and normally connected to the high-voltage winding of the transformer for voltage control;        comprises oil which may entail a non-negligible fire hazard in connection with internal partial discharges, so-called corona, sparking in on-load tap changers and other fault conditions;        comprises normally a monitoring device for monitoring gas dissolved in the oil, which occurs in case of electrical discharges therein or in case of local increases of the temperature;        comprises oil which, in the event of damage or accident, may result in oil spillage leading to extensive environmental damage.        